1. Field of the Invention
The present invention relates to a system and method for characterizing the quality of a link between nodes in a wireless communication network, such as a wireless ad-hoc peer-to-peer network, with minimum network overhead. More particularly, the present invention relates to a system and method efficiently characterizing the bi-directionality of links between nodes in a wireless network, such as a wireless ad-hoc peer-to-peer network.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at a first node to communicate simultaneously with several other nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in conventional ad-hoc networks, further enable the mobile nodes to access a fixed network and thus communicate with other types of user terminals, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, issued on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, issued on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, issued on Mar. 29, 2005, the entire content of each being incorporated herein by reference.
As can be appreciated by one skilled in the art, since certain nodes of the ad-hoc network are mobile, it is necessary for the network to maintain connectivity with those nodes. Transmitted data packets typically “hop” from mobile device to mobile device, creating a transmission path, or route, until reaching a final destination. However, transmission paths between mobile devices are often subject to change as devices move, therefore ad-hoc network communication must be able to adapt to achieve optimum performance while addressing the limited capabilities and capacities of mobile individual devices. Hence, in order to be able to select optimum paths or links between nodes in order to maximize performance of the network, it is necessary for the network and nodes to be capable of assessing the quality of the links between nodes.
In a typical wireless network, the measurement of quality of link between two nodes A and B is done separately on both the nodes and the information is never exchanged between the nodes. This may result in a situation where node A interprets the link between itself (node A) and node B as having high quality, while node B might have a totally opposite view (i.e., node B may interpret the link between itself and node A as having poor quality), which may lead to inefficient routing as different nodes in the network can compute different routing metrics depending upon their connection with nodes A and B. An example of a technique for computing such routing metrics is described in a U.S. Provisional Patent Application Ser. No. 60/476,237 referenced above. Hence, since the traffic between nodes usually flows in both directions (e.g., in TCP or IEEE 802.11 MAC networks), it is always better for all nodes to have a uniform view of a link between nodes.
In a typical wireless network, the bi-directionality of a link between two nodes is tested in the following two ways:
Hello Messages
Initially, all nodes send hello messages at regular interval notifying their neighbors about their presence. The hello message contains the address of the sender and can have other information as can be appreciated by one skilled in the art. After receiving hello message from the neighbors, each node modifies its hello message to include the list of neighbors (“heard hello list”) that it recently heard. When a node receives a hello message with its address in the “heard hello list”, it determines that there is a bidirectional link between itself and the sender. Several variations to this approach are possible, such as sending only partial list of nodes that can be heard in place of sending the complete list as described, for example, in the RFC 3684 entitled “Topology Dissemination Based on Reverse-Path Forwarding (TBRPF) which can be found at http://www.ieff.org/rfc/rfc3684.txt, the entire content of which is incorporated herein by reference. Other variations would include indicating whether the link between the neighbor advertised is deemed bidirectional or unidirectional, using Routing Advertisements or Neighbor Advertisements in place of hello message, and so on.
Although this hello message technique can be somewhat effective, the technique suffers from the following disadvantages. First, the technique is slow, since it takes a 3-way handshake to determine the bi-directionality of a link. The technique is also inefficient, because sending the neighbor addresses in the hello message can consume a significant amount of bandwidth, especially in denser environments. Furthermore, the technique can be faulty, because receiving just a hello message is no indication of the quality of the link. In wireless channels, the reception of short, intermittent messages is no guarantee that the link can be used to effectively deliver traffic, which is comprised of longer packets sent at a higher rate.
Unicast Packet
In place of sending hello message periodically, each node can determine the bi-directionality of a link to a neighbor after receiving a response to a unicast message from the other node. The response can be of the form of a “Clear To Send” (CTS) after sending a “Request To Send” (RTS), or in the form of an Acknowledgement (ACK) after sending a data message. These responses are typical of CSMA/CA or 802.11 MAC protocols.
Although this technique also can be effective, the technique suffers from the following disadvantages. First, this is a reactive approach and makes it necessary for the nodes to send some unicast message before determining the bi-directionality. Also, the technique is faulty, because receiving just an ACK or CTS message is no indication of the quality of link. Again, in wireless channels, the reception of short, intermittent messages is no guarantee that the link can be used to effectively deliver traffic.
Accordingly, a need exists for an improved system and method for characterizing the quality of a link, in particular, a bi-directional link, between nodes in a wireless communication network, such as a wireless ad-hoc peer-to-peer network, with minimum network overhead.